1. Technical Field
Illustrative embodiments described in this patent specification generally relate to an image forming apparatus in which compressed air is used to separate a recording medium from a component therein, and more particularly to an image forming apparatus employing an electrophotographic method in which heat is used to fix an image onto a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
An example of a widely-used system employed in the fixing device includes a heat roller fixing system, in which a fixing roller having a halogen heater or the like therein and a pressing roller pressed against the fixing roller to form a nip between the fixing roller and the pressing roller are provided so as to fix the toner image onto the sheet at the nip using heat and pressure while the sheet is sandwiched and conveyed by the rollers.
There is also known a belt fixing system, in which a seamless fixing belt is wound around a fixing roller and a heat roller having a halogen heater or the like therein. A pressing roller is pressed against the fixing roller with the fixing belt therebetween to form a nip between the pressing roller and the fixing belt. The toner image is fixed onto the sheet at the nip using heat and pressure while the sheet is sandwiched and conveyed by the fixing belt and the pressing roller.
In the above-described fixing systems using heat, melted toner of the heated toner image so as to be fixed onto the sheet can inadvertently contact the fixing roller or the fixing belt. In order to prevent adhesion of the toner and the sheet to the fixing roller or the fixing belt, a surface of the fixing roller or the fixing belt is often coated with a fluorinated resin having good releasability, and a separation pick is used to separate the sheet from the fixing roller or the fixing belt. However, because the separation pick contacts the fixing roller or the fixing belt, use of the separation pick tends to damage the surface of the fixing roller or the fixing belt, thereby causing undesired lines in a resultant image.
In general, a fixing roller employed in a monochrome image forming apparatus includes a metal roller with a surface coated with Teflon®. Accordingly, the surface of the fixing roller is not usually damaged by the separation pick, thereby extending the product life of the fixing roller. By contrast, in order to enhance coloring of full-color images, a surface layer of a fixing roller employed in a full-color image forming apparatus is formed of fluorine-coated silicone rubber such as a PFA tube having a thickness of several dozen microns or silicone rubber, with oil applied to the surface. However, the surface layer of such a fixing roller is easily damaged due to its softness, and therefore means such as the separation pick that contacts the fixing roller to separate the sheet from the fixing roller is not usually used for full-color image forming apparatuses in recent years. Instead, contactless means that separates the sheet from the fixing roller without contacting the fixing roller are often used.
However, use of the contactless means tends to cause sheet jam due to a sheet inadvertently getting wound around the fixing roller after the toner image is fixed to the sheet due to increased adhesion between the toner and the fixing roller. In particular, multiple toner layers of different colors are superimposed one atop the other in full-color image formation, thereby more often causing sheet jam due to increased adhesion between the toner and the fixing roller.
Examples of well-known means for sheet separation employed in full-color image forming apparatuses in recent years are as follows: (i) a contactless separation plate extending parallel to a longitudinal direction of a fixing roller (or a width direction of a fixing belt), with a small gap of about from 0.2 mm to 1 mm between the separation plate and the fixing roller (or the fixing belt); (ii) contactless separation picks provided at predetermined intervals, with a small gap of about from 0.2 mm to 1 mm between each of the separation picks and a fixing roller (or a fixing belt); and (iii) a self-stripping system that causes the sheet to separate from the fixing roller (or the fixing belt) by itself by taking advantage of the stiffness of the sheet and the elasticity afforded by the curve of the fixing roller (or the fixing belt).
In addition, a guide plate is provided that guides the sheet to an exit of the fixing device. The guide plate is disposed across a slight gap from the fixing roller (or the fixing belt). Consequently, a thin sheet, a sheet having a small margin at a leading edge thereof, or a sheet bearing a solid image such as a photograph thereon is not separated from the fixing roller (or the fixing belt) while being conveyed through the gap, thereby causing sheet jam. Alternatively, the sheet may contact the separation plate or the separation pick, thereby causing sheet jam.
In order to solve the above-described problems, use of compressed air ejected from a nozzle as an auxiliary for the contactless separation means has been proposed. Specifically, compressed air is ejected from the nozzle toward a sheet separation position so that the sheet is separated from the fixing roller or the fixing belt. Examples of proposed configurations using the nozzle include, but are not limited to, a configuration in which an exhaust duct having an exhaust valve is connected near an ejection opening of a duct connected to an ejection nozzle to prevent unnecessary ejection of compressed air from the nozzle to the fixing roller, and a configuration in which a channel through which compressed air flows is provided where a main body of the image forming apparatus engages a positioning pin for the fixing device.
Thus, a compressed air generator (hereinafter referred to as a compressor) and a pneumatic duct system, starting from the compressor and ending at the nozzle, that controls the compressed air generated by the compressor are provided to the image forming apparatus employing the sheet separation system, in which the compressed air is ejected from the nozzle to the sheet separation position so as to separate the sheet from a fixing member such as the fixing roller or the fixing belt without contacting and damaging the fixing member. In the pneumatic duct system, an air filter that removes water droplets and foreign substances from the compressed air, an air tank that suppresses a pressure change in the compressed air, a pressure control valve serving as a mechanical control valve that controls the compressed air in the air tank to have a predetermined pressure, an electromagnetic valve serving as an electronic control valve that controls ejection and non-ejection of the compressed air, and a pneumatic member such as the nozzle are generally provided downstream from the compressor in a direction of flow of the compressed air. The above-described components are connected with tubes to achieve the pneumatic duct system.
Moist air compressed by the compressor has a higher temperature than dry air. Therefore, when the compressed air is cooled while passing thought the pneumatic duct system, supersaturated moisture turns into water droplets. Further, in the above-described sheet separation systems, the compressed air is ejected from the nozzle into the atmosphere. As a result, the pressure of the compressed air in the pneumatic duct system is decreased, adiabatic expansions occurs, and the temperature of the compressed air is decreased. The temperature decrease also produces water droplets within the pneumatic duct system.
Because the water droplets thus generated adversely affect the components of the pneumatic duct system, means for removing the water droplets is often provided in the pneumatic duct system including the compressor. For example, the water droplets generated upstream from the air filter in the direction of flow of the compressed air accumulate in the air filter. Therefore, the accumulated water droplets are discharged from the pneumatic duct system by a removal mechanism. In addition, because the air tank is formed of metal and has a larger contact surface area with the compressed air, the compressed air tends to be cooled, thereby easily generating water droplets. The water droplets thus generated adhere to inner wall surfaces of the air tank and tubes provided downstream from the air tank in the direction of flow of the compressed air, accumulate on the bottom of the air tank, and are discharged from the pneumatic duct system by the removal mechanism. The removal mechanism is provided to the bottom of the air filter and the air tank where the water droplets accumulate, and discharges the water droplets from the pneumatic duct system by opening valves, which are main components of the removal mechanism. The valves are opened either manually, automatically using a pressure difference, or electrically using electromagnetic valves.
The larger the pressure and flow of the compressed air generated by the compressor, the more water droplets generated. When using a larger-size compressor that outputs 1 kW or more, a dehumidification device called an air dryer is provided immediately downstream from the compressor in the direction of flow of the compressed air. The air dryer forcibly cools water vapor included in hotter compressed air generated by the compressor so that the water vapor turns into water droplets. The water droplets are then captured by a water separator to be discharged from the pneumatic duct system, absorbed by an absorption agent, or separated from the compressed air using a hollow-fiber filter to be discharged from the pneumatic duct system so as to dehumidify the pneumatic duct system.
However, high-performance air dryers are costly, and larger electric energy consumption is required for cooling the water vapor. In addition, use of the hollow-fiber filter requires a higher pressure, equal to or greater than 0.2 Mpa. In general, an amount of pressure required for sheet separation in the image forming apparatus is as small as from 0.05 MPa to 0.2 MPa, and a required amount of flow of the compressed air is small. Therefore, a compact compressor that outputs 200 W or less is generally employed in the image forming apparatus. Because the amounts of pressure and flow of the compressed air generated by the compact compressor are smaller compared to the previously-described larger-size compressor, fewer water droplets are generated in the compressed air. Thus, it is not practical to employ a high-performance air dryer in the image forming apparatus.
In order to efficiently eject the compressed air from the nozzle, it is preferable that the pneumatic duct system that connects the air tank, the electromagnetic valve, and the nozzle have low resistance and volume. Therefore, in an image forming apparatus including a low-output compressor, a water droplet collector such as an air filter having larger resistance and volume is not often provided in the pneumatic duct system.
In the image forming apparatus in which sheet separation is performed by ejecting the compressed air, the compressed air is ejected each time the sheet passes. Therefore, the water droplets adhering to the inside of the pneumatic duct system, particularly a part of the duct system provided downstream from the air tank in the direction of flow of the compressed air, are discharged from the nozzle. Fewer water droplets are ejected from the nozzle for each ejection, thereby preventing blots on the sheet.
However, more water droplets pass through the duct system during a longer operating period under higher temperature and humidity conditions in which the air has a moisture content. Consequently, the water droplets often accumulate on parts of the duct system having a larger resistance, such as joints and branching portions between pneumatic devices. The water droplets thus accumulated are ejected from the nozzle in large numbers in any given ejection. Consequently, the water droplets thus ejected adhere to the fixing member or the sheet. Further, the water droplets generated within the pneumatic duct system adversely affect operation of the pneumatic devices such as the electromagnetic valve and the pressure control valve, thereby shortening the life of the pneumatic duct system.